The present invention relates to an electroluminescent semiconductor device of the type which, starting from a monocrystalline substrate, is provided successively with: an n-type gallium nitride layer, an active gallium nitride layer doped at least to complete compensation of the natural donor impurities with acceptor impurities and a surface electrode which is in contact with the said active layer, the said device comprising in addition means to contact the said n-type layer.
The invention also relates to a method of manufacturing such a device.
It is known that electroluminescent semiconductor devices are used especially to display data in various forms and are provided with punctiform, digital or alphanumerical indicators in accordance with whether they have a punctiform luminescent region, a luminescent region having segments or a dot matrix. Various types of devices have been proposed as far as mode of operation is concerned.
Up to now, devices having pn-junctions have been preferred due to their good luminous efficiency, their rigidity, their life and their comparatively low polarization voltage compared with other optical display devices using different techniques.
The semiconductor materials most frequently used to apply such devices are gallium arsenide, gallium arsenide phosphide, aluminium gallium arsenide and gallium phosphide, which permits a light emission in radiation ranges which are between the near infra-red and the green, such as red, orange and yellow. It is known that the forbidden bandwidth of a semiconductor inter alia determines the maximum radiation energy which it can emit. On the other hand, in accordance with the nature and the concentration of certan impurities incorporated in the semiconductor, the energy of the radiation which it can emit may be reduced and thus radiations may be obtained the color of which approaches that of infra-red.
It thus has been endeavored to expand the range of semiconductor materials used so far in such manner that the spectrum of visible radiation is fully covered and to produce light in the blue, the violet and even in the near ultra-violet.
Recently, gallium nitride has been suggested as one of the semiconductor materials which are suitable for this purpose.
At the moment it is not possible to obtain monocrystalline gallium nitride other than by an epitaxial method by which the synthesis of the material is simultaneously performed at a temperature which is much lower than the melting point of said material. In fact it is known that gallium nitride has a clear tendency to decomposition into its element when it is heated at a high temperature, for example above 800.degree. C.
Associated with this tendency to dissociation is the fact that during the synthesis of GaN in the vapor phase, by reaction of gallium monochloride with ammonia gas, the whole immersed in a carrier gas, the material obtained without intentional doping (the materials used in the reaction being as pure as possible) is always of the n-type and wth a high concentration of donors originating from centers which to all probability are related to "nitrogen vacancies". The donor centers will hereinafter be referred to as natural donor impurities.
It is known, especially by a publication of J. I. Pankove, entitled "Luminescence in GaN" published in "Journal of Luminescence", volume 7, 1973, pp. 114, 126, that by the introduction of a dopant, such as zinc, cadmium, magnesium, lithium or beryllium, the natural conductivity of the n-type of the material can be compensated. It is possible to obtain substantially insulating gallium nitride by using a sufficiently high concentration of the said dopant. Until now it has not been possible to obtain gallium nitride in this manner having a large p-type conductivity. As a result of this, the electroluminescent GaN devices of the known type mainly have an M-i-n structure, that is to say: metal/GaN of substantially insulating type/GaN of the n-type.
A close investigation of the prior art as described especially in the above-mentioned publication "Journal of Luminescence" reveals that the results obtained with the electroluminescent gallium nitride devices with M-i-n structures, taken on an average, remains comparatively moderate with respect to luminous efficiency. In particular, the operating characteristics of the devices usually are very different for each individual device and for each individual material sample. The lack of reproducibility relates especially to the polarization voltages of the devices at a given current density and the homogeneous character of the luminescence of the active zone, said characteristics being without apparent correlation to the thicknesses of the layers used. The color of the radiation itself does not seem to be apparently related to the growth conditions of the semiconductor material.
In another paper by J. I. Pankove entitled "Low Voltage Blue Electroluminescent in GaN" published in "IEEE Transactions on Electron Devices", volume ED 22, No. 9, Sept. 1975, p. 721, the author remarks that for reasons which are still obscure the incorporation of zinc in the crystal seems to increase gradually during the growth, although the partial pressure of zinc in the atmosphere of the reaction was apparently maintained constant. The complete compensation of the zinc-doped layer thus occurs only towards the center of the thickness thereof, which will cause uncertainty regarding the thickness of the layer parts of the n-type on the one hand and of the substantially insulating type on the other hand, the concentration variation of the impurity as a function of the thickness of the growing material occurring in a nearly uncontrollable manner, especially at the precise depth level where the layer becomes insulating.
As regards the range of the wavelengths which the electroluminescent GaN devices can emit, it is known that this can vary with the nature and the concentration of the dopant incorporated in the material so as to make the active layer substantially insulating. For example, in the case in which the said dopant is zinc, a luminescence has been observed in some cases the intensity of which culminates in the proximity of a wavelength of 440 nm, for example, a blue light with an energy of approximately 2.8 eV, or, in the proximity of 500 nm, green light, with an energy of 2.5 eV, or, in the proximity of 565 nm, yellow light with an energy of 2.2 eV.
Although it has been observed here and in general that the color obtained depended on the concentration in such manner that a comparatively weak concentration corresponds to an emission of blue color and a comparatively strong concentration corresponds to an emission of yellow color, this has so far not been confirmed by experiments. The lack of reproducibility of the experiments stated in the prior art does not permit drawing a conclusion as regards the experimental conditions in which one of the luminescent wavelengths can be obtained in a regular manner.